Such a drain trap normally comprises a plurality of parts, namely a strainer for the retention of solids and at least two further parts that together form a gas and odor trap. One of these latter parts is provided with an outlet connected to a drain pipe. Reference in this connection may be made to my copending application Ser. No. 894,488 filed Apr. 7, 1978, now U.S. Pat. No. 4,198,717.
It is essential that such an arrangement have a relatively large flow cross-section at all locations along the path followed by liquid passing through it. For this reason the trap usually has a housing of considerable horizontal or vertical extent in order to accommodate the various parts of the structure without excessively restricting the flow. Thus when the strainer is arranged above the trap the device tends to be relatively tall, whereas when it is arranged next to the trap it tends to be relatively long.
Furthermore, in such arrangements the strainer is normally a separate element, so that the device must be cast in several pieces which are subsequently interconnected. As the applicable plumbing codes invariably require that a certain minimum flow cross-section be maintained throughout such traps, their design becomes relatively complex, especially when a compact structure is desired.